THIN FILM GAS SENSOR AND METHOD OF FABRICATION THEREOF

Abstract: An integrated monolithic gas sensor comprises a substrate and thin films deposited on this substrate. The thin films include a thin film electrically conductive heating element, a thin film conductive reference electrode, and a second thin film conductive electrode, these electrodes and heating element being electrically isolated from each other. A thin film ionic conductor and a thin film reactive gas sensitive layer are placed between the reference electrode and the second conductive electrode to form an electrolytic cell in which an electrolytic reaction including as reagent the gas to be detected produces between the two conductive electrodes an electromotive force indicative of the concentration of the gas. Also deposited is a micro-thermometer formed of a thin film wire having a temperature-dependent resistance. When the gas to be detected is CO2, the ionic conductor may comprise NASICON of formula Na3Zr2Si2PO12, and the reactive gas sensitive layer may comprise Na2CO3.

Description: THIN FILM GAS SENSOR AND METHOD OF FABRICATION THEREOF BACKGROUND OF THE INVENTION 1. Field of the invention: The present invention relates to an integrated monolithic thin film gas sensor capable of continuously detecting and monitoring low concentration of gas with a sensitivity of 100 p.p.m. or less. The present invention further relates to a method of fabricating this sensor, with thin film fabrication techniques. 2. Brief description of the prior art: Many techniques have been proposed for detecting concentrations of gases such as C0 2 in the air. For example, spectroscopic techniques including infrared and photoacoustic spectroscopies carry out detection through electronic transitions produced in the gas when illuminated. Another example is concerned with an electrochemical cell provided with an electrode chemically sensitive to the gas to be sensed. The gas sensors that have been constructed on the basis of these prior art techniques are bulky, fragile, not integrable with the electronic circuit through which they are controlled, and, generally, they are not adapted to inexpensive mass production. In particular, spectroscopic sensors are as a whole expensive and quite fragile. Another prior art gas sensor is described in U.S. patent N° 4,668,374 granted to Bhagat et al. on May 26, 1987. More specifically, this patent discloses a fast response gas sensor fabricated through microelectronics technology to form multiple thin film solid-electrolyte pump and sense cells within a hermetically sealed sensor cavity. This prior art sensor requires an airtight chamber which must be free of any chemical and any source of current leakage. The electrolyte is in the form of a membrane, and the response time is related to the thickness of the electrolyte, and thus to the thickness of the membrane. To obtain very fast response times, the membrane of the electrolyte must be so thin that it suffers from mechanical integrity; the resulting membrane is fragile and can rupture easily in normal service to render the sensor inoperative. Moreover, the sensor of Bhagat et al. is designed to be used in an ambient temperature of 300 "C, and comprises an ionic conductor which, at that temperature, has a sufficient conductivity and does not suffer from interference due to water adsorbed at the surface of the sensor. Therefore, a drawback of the sensor of Bhagat et al. is that it cannot be used at room temperature. Also, nothing is disclosed regarding methods to increase sensitivity of the gas detection, or to integrate the sensor with an electronic control circuit. OBJECTS OF THE INVENTION An object of the present invention is therefore to overcome the above discussed drawbacks and to provide an integrated monolithic thin film gas sensor which is free of hermetic "reference" cavity and is mechanically robust. Another object of the invention is to provide an integrated monolithic thin film gas sensor which is compatible with mass production. A third object of the present invention is to provide a monolithic thin film gas sensor comprising an integrated thin film heating element for heating the sensor and improve the detection performance while reducing the susceptibility to interference from gases other than the target one. A fourth object of the invention is to provide a monolithic thin film gas sensor comprising an integrated thermometer to enable measurement and control of the temperature of operation of the sensor. A further object of the present invention is to provide integrated monolithic thin film gas sensors which can be connected in cascade to improve sensitivity of gas detection. A sixth object of the subject invention is to provide an integrated monolithic gas sensor comprising a thin film ionic conductor, and a "dry" fabrication technique for producing this thin film ionic conductor having an improved ionic conductivity. A still further object of the invention is to provide a thermal process which significantly improves ionic conduction of the thin film solid state electrolyte. SUMMARY OF THE INVENTION More specifically, in accordance with the present invention, there is provided a monolithic gas sensor comprising first and second electrodes each made of a film of electrically conductive material, a reactive layer made of a film of material sensitive to the gas to be detected, and an ionic conductor being under the form of thin film to efficiently conduct ions therein. The first and second electrodes, the reactive layer, and the thin film ionic conductor are deposited on each other to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between the first and second electrodes. In accordance with a preferred embodiment of the invention, the monolithic gas sensor comprises: a substrate; a thin film heating element made of electrically conductive material deposited on the substrate; a thin film of dielectric material deposited on the thin film heating element; a reference electrode consisting of a thin film of electrically conductive material deposited on the thin film of dielectric material; a thin film ionic conductor deposited on the thin film reference electrode; a reactive gas sensitive layer consisting of a thin film of reactive gas sensitive material deposited on the thin film ionic conductor; a second electrode formed of a thin film of electrically conductive material deposited on the thin film reactive gas sensitive electrode; and a thin film micro-thermometer consisting of a portion of the thin film of electrically conductive material constituting the reference electrode, the thin film portion forming the micro- thermometer having a temperature-dependent resistance than can be converted to temperature. In accordance with another preferred embodiment of the present invention, the monolithic gas sensor comprises: a substrate; a thin film of dielectric material deposited on the substrate; a thin film of electrically conductive material deposited on the thin film of dielectric material and divided into four separate thin film portions respectively forming (a) a thin film reference electrode, (b) a second electrically conductive electrode, (c) a thin film heating element, and (d) a thin film microthermometer; a thin film ionic conductor deposited onto the thin film reference electrode; and a reactive gas sensitive thin film layer deposited on the second electrically conductive electrode and on the thin film ionic conductor. When the gas to be detected is C0 2 , the thin film ionic conductor may comprise NASICON of formula Na 3 Zr 2 Si 2 P0 12 , and the reactive gas sensitive thin film layer may comprise Na 2 C0 3 . The monolithic gas sensor in accordance with the subject invention may comprise a plurality of said electrolytic cell mounted in cascade in order to improve gas detection sensitivity of the sensor. The present invention further relates to a method of fabricating a monolithic gas sensor, comprising the step of depositing onto a substrate (a) first and second thin film electrodes made of electrically conductive material, (b) a reactive thin film layer made of material sensitive to the gas to be detected, and (c) an ionic conductor under the form of thin film to efficiently conduct ions therein, wherein the depositing step comprises stacking the first and second thin film electrodes, the reactive thin film layer, and the thin film ionic conductor to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produce an electromotive force between the first and second thin film electrodes. In accordance with a preferred embodiment of the invention, the method of fabricating a monolithic gas sensor further comprises the step of depositing on the substrate a thin film heating element made of electrically conductive material, and the step of depositing on the substrate a micro- thermometer made of a thin film of electrically conductive material having a temperature-dependent resistance which can be converted to temperature. Patterning of the first and second thin film electrodes, the reactive thin film layer, the thin film ionic conductor, the thin film heating element, and the thin film micro-thermometer can be carried out by means of proximity masking during the thin film deposition process or by means of laser ablation following this thin film deposition process. In accordance with another preferred embodiment of the method of fabricating a monolithic gas sensor, rapid thermal annealing is carried out on the thin film ionic conductor to favor crystalline microstructure formation and thereby improve ionic conductivity of this thin film ionic conductor. The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the appended drawings: Figure 1 is a side elevational view of a first preferred embodiment of an integrated monolithic thin film gas sensor in accordance with the present invention, capable of sensing CO., and comprising integrated thin film ionic conductor, heating element and micro-thermometer; Figure 2 is a top plan view of the thin film gas sensor of Figure 1; Figure 3 is a side elevational, cross sectional view, taken along line 3-3 of Figure 4, of a second preferred embodiment of the integrated monolithic thin film gas sensor in accordance with the present invention, capable of sensing C0 2 and including integrated thin film ionic conductor, heating element and micro-thermometer; Figure 4 is a top plan view of the thin film gas sensor of Figure 3; and Figure 5 is a side elevational view showing two gas sensors as illustrated in Figures 3 and 4 mounted in cascade. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first preferred embodiment of the integrated monolithic thin film C0 2 sensor in accordance with the present invention is generally identified by the reference 11 in Figures 1 and 2 of the appended drawings. As illustrated in Figure 1, the sensor 10 comprises a stack of six thin films (see 12-17) deposited onto a substrate 11. Substrate 11: The substrate 11 is made of a piece of crystalline silicon, glass, metal, ceramic, or plastic material capable of resisting to high temperatures. Fabrication and operation of the thin film Cθ 2 sensor 10 requires a substrate 11 of which the material is resistant to temperatures higher than 200 "C, to water and water vapour, to any organic or inorganic chemical contaminants, and to dust accumulation. Ideally, the elastic constants (Young's modulus and Poisson's ratio) and the thermal expansion coefficient of the material constituting the substrate 11 are nearly those of silicon or quartz to avoid cracking due to temperature cycling and ultimately problems related to reliability and short-lifetime of the sensor 10. Heating element 12: The first thin film 12, deposited on the top surface 18 of the substrate 11, is made of metal such as tungsten or of highly conductive silicon to form a thin film heating element. Heating element 12 is an electrically conductive thin film supplied with a current I. The thickness, width and length of the thin film heating element 12 are adjusted to obtain a resistance R adapted to an external current source supplying the current I. Power (I 2 R) is dissipated in the resistive heating element 12 by the Joule effect to thereby heat the sensor 10. The sandwich geometry of the thin film sensor 10 of Figures 1 and 2 requires no electrical insulation between the heating element 12 and the substrate 11 as long as the material constituting this substrate 2 is not substantially conductive; it is the case for a substrate 11 made of silicium, glass, plastic or ceramic material. When the substrate 11 is made of metal and is therefore electrically conductive, an insulating thin film (not shown in Figures 1 and 2) is required to prevent the heating current, normally flowing through the heating element 12, to deviate in the substrate 11. Obviously, the current flowing through the substrate 11 does not contribute in heating the sensor 10. Thin film of dielectric material 13: The second thin film 13, deposited on the thin film heating element 12, consists of dielectric material such as Si0 2 to electrically insulate the thin film heating element 12 from the thin film reference electrode 14. Other insulating dielectric materials, for example Si 3 N A , could also be used as the material of the thin film of dielectric material 13. Other functions of the thin film of dielectric material 13 are to ensure good adhesion between the thin film heating element 12 and the subsequent thin films 14-17, and to provide a diffusion or alloying barrier between the metallic heating element 12 and the metallic reference electrode 14. The thin film 13 should have good thermal properties, in particular a good thermal expansion coefficient, and should be chemically neutral so as not to react with gases to which the sensor 10 is exposed and so as to protect the substrate 11 from these gases. Reference electrode 14: The reference electrode 14 is metallic and formed of a thin film of pure metal such as platinum or other noble metal deposited on the thin film 13 of dielectric material. As will be seen in the following description, the metallic thin film 14 is also used to form the integrated micro-thermometer. As the function of the thin film reference metallic electrode 14 is to collect electric current, its thickness must be sufficient to make this electrode continuous (with no electrical interruptions) and conductive with a series resistance smaller than 10 Ω. The thin film reference electrode 14 should not contact any other metallic thin film of the sensor 10, including the second metallic electrode 17 and the heating element 12. Ionic conductor 15: A further thin film 15 of NASICON (Na Super Ionic Conductor) , of formula Na 3 Zr 2 Si 2 P0 12 , is deposited on the thin film reference electrode 14 to form a thin film ionic conductor. The thin film ionic conductor 15 may have a homogeneous composition, or may include a plurality of layers which through their composition and/or structure serve to act as passivation, stabilisation and/or diffusion-barrier layers. Reactive CO.. sensitive layer 16: A further thin film, deposited on the thin film ionic conductor 15, is made of sodium carbonate alone or stabilized by barium carbonate to form a reactive thin film C0 2 sensitive layer 16. Second metallic electrode 17: A second metallic electrode 17 is made, as the reference electrode 14, of a thin film of platinum or other noble metal deposited on the sodium carbonate alone or stabilized by barium carbonate, of the reactive thin film C0 2 sensitive layer 16. Integrated micro-thermometer: A micro-thermometer 19 is integrated to the thin film C0 2 sensor 10. This micro-thermometer 19 comprises a portion 20 of the thin film of dielectric material 13 deposited onto the thin film heating element 12, and a portion 21 of the metallic thin film 14 deposited onto the thin film portion 20. As illustrated in Figure 2, the thin film portion 21 forms an elongate wire 22 having a temperature- dependent resistance. Mechanism of detection of sensor 10: The thin film ionic conductor 15 is a rather porous structure which is rich in sodium (Na) . In the presence of air which contains about 16% of oxygen (0 2 ) and some humidity, sodium reacts with oxygen to form Na 2 0 at the surface of the thin film ionic conductor 15, especially in the proximity of the interface with the "Pt" reference electrode 14, also known as the cathode of the electrolytic cell. Oxygen therefore plays a role in the chemical reaction of concern and is sometimes called the driving species or element in this chemical reaction. Interposed between the other "Pt" thin film metallic electrode 17, or anode, and the thin film ionic conductor 15 is the thin film CO z sensitive layer 16 made of sodium carbonate (Na 2 C0 3 ) alone or stabilized by barium carbonate. The difference in the electronegativities of the two substances, Na 2 0 and Na 2 C0 3 , gives rise to an electromotive force (EMF) between the thin film metallic electrodes 14 and 17. Mobile ions are subjected to the field produced by this electromotive force and drift along the field lines, thereby generating an ionic current. It should be noted here that NASICON is electrically insulating, so that electrons stay-put and do not travel in the field, but can be collected at the conductive thin film metallic electrodes 14 and 17 and circulated in external electric circuits such as resistive and capacitive loads, voltmeters, ammeters, etc. Under normal conditions, an equilibrium exists between oxidised sodium, and ionic sodium in the thin film ionic conductor 15 (solid electrolyte) . Sodium ions can also travel easily within the electrolyte by a special mass transport mechanism through the crystalline structure. Sites in the crystal lattice at which only sodium ions can sit are connected together to form paths or channels capable of conducting the sodium ions. The to and fro coupled rocking of the stacked oxide tetrahedra around zircon or silicon atoms "push" the sodium ions along these paths or channels and contribute to the "super ionic conductivity" referred to in the acronym NASICON (Na Super Ionic Conductor) . In the presence of carbon dioxide, a chemical reaction occurs in which sodium oxide is "reduced" by the carbon dioxide (C0 2 ) . This reduction causes sodium ions to be "liberated" from the oxide, to travel across the NASICON and react to form sodium carbonate (Na 2 C0 3 ) . The ionic transport generates the current and the electromotive force which can be detected by external circuits. The sodium ions have such high mobility, and such a short distance (in the thin film 15) to travel that the reaction is almost instantaneous. It has been observed in laboratories fast response times of less than a second, this compared to several minutes in bulk sensor devices operating at much higher temperatures. For a C0 2 sensitive thin film layer 16 made of sodium carbonate and a thin film ionic conductor 15 made of NASICON, the C0 2 detecting electrolytic cell of the sensor 10 is made of a combination of Na 2 C0 3 and of NASICON (Na 3 Zr 2 Si 2 P0 12 ) and the C0 2 detecting mechanism can therefore be described by the following electrolytic cell: Pt, C0 2 , 0 2 / Na 2 C0 3 // NASICON / 0 2 , Pt The anodic reaction is: Na 2 C0 3 > 2 Na + + C0 2 + h O→, + 2 e " The cathodic reaction is: 2 Na + + 0 2 + 2 e " Na 2 0 The global reaction is : Na 2 C0 3 <===> Na 2 0 + C0 2 The electromotive force of this cell is measured between the thin film metallic electrodes 14 and 17 and is proportional to the concentration of C0 2 in the air. The electrochemistrybehindthisprinciple of operation is well established and accordingly, will not be further described in the present disclosure. It is similar to that used in some commercially available sensors of higher cost (based on sintered ceramic plates of solid ionic conductors, and on thick metal electrodes pressed in contact) , and of lesser performance. The integrated thin film heating element 12 is supplied with electric current to heat the sensor 10 in order to minimize water absorption at the surface of the electrolyte thin film 15 (above water's boiling point (100 °C) water (H 2 0) is driven off) . The heating element 12 improves the C0 2 detection performance (ionic conductivity which depends on collective crystalline vibration is increased) and reduces the susceptibility of the sensor 10 to interference from gases other than C0 2 . Since the behavior of the sensor 10 is dependent on the temperature, the temperature-dependent resistive wire 22, made of platinum or other noble metal, of the micro-thermometer 19 is used to measure the temperature of operation of the sensor 10 and to control supply of current I to the thin film heating element 12. Temperature control of the sensor 10 is thereby carried out. Fabrication of sensor 10: The different steps conducted during fabricationn <of the thin film C0 2 sensor 10 will now be described. The surface 18 of the substrate 11 is first chemically etched to clean this surface 18 prior to thin film deposition. The substrate 11 must be properly cleaned to avoid organic and inorganic contamination (to levels of p.p.m.) of the thin films to be deposited thereon. Such contamination may lead to reliability problems such as slow degradation of the sensor 10. Tungsten or other refractory metal is deposited onto the etched surface 18 of the substrate 11 to form the thin film heating element 12. The heating element 12 can be deposited by thin film techniques including RF reactive magnetron sputtering, reactive evaporation in an ultra high vacuum chamber, electroplating, electroless plating, screen printing, plasma enhanced or normal chemical vapour deposition, etc. The thin film heating element 12 may also be produced by forming on the surface 18 of a silicon substrate 11 a thin film of highly conductive silicon by means of conventional techniques. The thin film of dielectric material 13 is then formed by depositing Si0 2 on the thin film heating element 12 either by RF reactive magnetron sputtering, or plasma enhanced or normal chemical vapour deposition. Platinum or another noble metal is deposited onto the thin film of dielectric material 13 to produce the thin film reference electrode 14. Again, the metallic electrode 14 can be deposited by means of thin film techniques including reactive evaporation in an ultrahigh vacuum chamber, RF reactive magnetron sputtering, electroplating, electroless plating, screen printing, or plasma enhanced or normal chemical vapour deposition. Ionic conductor material (solid electrolyte) is sputtered onto the thin film reference electrode 14 to form the thin film ionic conductor 15. Sputtering of the ionic conductor material forming the thin layer 14 can be done from a single target of NASICON or by co-sputtering of two targets, which are ZrSi0 4 and Na 2 C0 3 . The sputtered thin film is amorphous. RF reactive magnetron sputtering is a technique that is increasingly used in the manufacture of microelectronics integrated circuit to deposit metallic thin films. Reference is made to the very recent "Handbook of Sputter Deposition Technology", by Wasa and Hayakawa, (1991, Noyes Publishing) , for a complete state-of-the-art expose of sputtering phenomena, sputtering systems, thin film families, micro-fabrication technology and even future processing and materials. It appears that the inventors of the present invention are the first to apply this technology to the production of thin film ionic conductors which are between 0.01 and 1 μm thick. Thin film ionic conductors have been produced in ultrahigh vacuum by sputtering constituent material in a reactive atmosphere with trace pure oxygen, using highly focused argon bombardment and high sputtering rates associated with magnetic plasma confinement at the sputtering cathode (known as magnetron sputtering) . Ultrahigh purity of the sputtered material(s) and, generally, of the sputtering process is required to avoid any contamination of the thin film as the structure builds up atom by atom. Rapid thermal annealing of the thin film ionic conductor 15 (and also of the sensor 10 completed up to this point) is carried out so as to obtain: (a) low mechanical stresses in the thin films during fabrication; (b) high ionic conductivity and hence lower sensor operation temperatures. Rapid thermal annealing or oxidation in forming gas (H 2 /N 2 ) favors formation of crystalline microstructures that improve ionic conductivity of the thin film; (c) good adhesion between the thin layers; (d) good chemical stability; (e) good crack resistance; and (f) good thermal expansion characteristics matched to those of the substrate and ultimately to the package. The advantage of a thin film ionic conductor 15 is the increase of mobility of ions Na + in thin film to thereby reduce the temperature of operation of the sensor. The thin film ionic conductor enables passage of the ions Na + while blocking the other ions. Sodium carbonate alone or stabilized by barium carbonate is then deposited on the thin film ionic conductor 15 to form the reactive thin film C0 2 sensitive layer 16. RF magnetron sputtering is probably the best method to deposit the thin film C0 2 sensitive layer 16. However, other methods such as sol-gel coating, reactive evaporation in an ultra high vacuum chamber and chemical vapour deposition, etc. can also be contemplated. Platinum or another noble metal is finally deposited onto the thin film C0 2 sensitive layer 16 to form the second thin film metallic electrode 17. It can be deposited by thin film techniques including reactive evaporation in an ultrahigh vacuum chamber, RF reactive magnetron sputtering, electroplating, electroless plating, screen printing, or plasma enhanced or normal chemical vapour deposition. To enable wiring of the sensor 10 using conventional bonding techniques, no material is deposited on surface portions 23 and 24 (Figure 2) of the thin film heating element 12, and surface portion 25 (Figure 1) of the thin film reference electrode 14. Electric wires 26 and 27 (Figure 2) can then be connected to surface portions 23 and 24 of the heating element 12, respectively, while electric wire 28 is connected to surface portion 25 of the thin film reference electrode 14. As no material is deposited on the metallic thin film electrode 17 and on the metallic thin film portion 21 of the micro-thermometer 19, a wire 29 is easily connected to the metallic electrode 17, and the micro-thermometer easily wired by means of electric wires 30 and 31. No material is deposited on surface portion 32 (Figure 1) of the reactive C0 2 sensitive layer 16 to increase the surface of contact of the C0 2 with this layer 16. Proximity masking can be used during deposition of each thin film 12-17 to produce the structure shown in Figure 1, comprising exposed surface portions 23 and 24 of the thin film heating element 12, exposed surface portion 25 of the thin film reference electrode 14, and exposed surface portion 32 of the reactive layer 16. One of ordinary skill in the art will also appreciate that the same proximity masking operations can be adapted to deposit simultaneously the thin film portions 20 and 21 forming the micro-thermometer 19. The C0 2 sensor 10 can also be fabricated through deposition of thin films 12-17 having a same area. Patterning of the thin film C0 2 sensor 10 and micro-thermometer 19 is then carried out by laser ablation to cut and remove selectively parts of thin films so as to expose the above mentioned surface portions 23, 24, 25 and 32, and to cut the thin film portions 20 and 21 of the micro-thermometer 19. This approach for cutting the stack of thin films 12-17 is necessary due to both the high solubility of the carbonaceous materials which are used, and to the electrochemical interference of water molecules penetrating the thin films. Finally, the substrate 11 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin film C0 2 sensor 10 is then packaged using conventional hybrid packaging techniques. The second preferred embodiment of the integrated monolithic thin film C0 2 sensor in accordance with the present invention is generally identified by the reference 40 in Figures 3 and 4 of the appended drawings. Substrate 41: As illustrated in Figures 3 and 4, this sensor 40 comprises a substrate 41 made of a piece of crystalline silicon, glass, metal, ceramic, or plastic material capable of resisting to high temperatures higher. Fabrication and operation of the sensor 40 requires that the material of the substrate 41 be capable of resisting to temperatures higher than 200 °C, to water and water vapour, to any organic or inorganic chemical contaminants, and to dust accumulation. Ideally, the elastic constants (Young's modulus and Poisson's ratio) and the coefficient of thermal expansion of the material forming the substrate 41 are nearly those of silicon or quartz to avoid cracking due to temperature cycling and ultimately problems of reliability and short-lifetime of the sensor 40. Thin film dielectric material 42: The first thin film 42, deposited on the substrate 41, consists of dielectric material such as Si0 2 . Use of other insulating materials, for example Si 3 N 4 , may also be contemplated. The functions of the thin film 42 are to ensure good adhesion between the substrate 41 and the subsequent thin films 43-45, to ensure electrical insulation between the substrate 41 and the metallic thin film 43, and to provide a diffusion or alloying barrier between the substrate 41 and the subsequent metallic thin film 43. The thin film 42 has good thermal properties, in particular a good thermal expansion coefficient, and is chemically neutral so as not to react with gases to which the device is exposed, and so as to protect the substrate 42 from these gases. Metallic thin film 43: The thin film 43 is metallic and made of pure metal such as platinum or other noble metal deposited on the thin film of dielectric material 43. The metallic thin film 43 is divided into four separate portions 46-49. Reference electrode 46: The first thin film portion 46 is a thin film reference electrode whose function is to collect electrical current. The thickness of the reference electrode 46 must be sufficient to make this electrode continuous (with no electrical interruptions) and conductive with a series resistance smaller than 10 Ω. It is important that the metallic electrode 46 contacts no other metallic thin film portions 37-39. Second metallic electrode 47: The second thin film portion 47 forms a second metallic electrode used, with the thin film reference electrode 46, to measure the electromotive force representative of the CO 2 concentration. Heating element 48: Thin film portion 48 defines an open (see 54) metallic peripheral loop constituting a heating element. This heating element 48 is supplied with a current I. The thickness, width and length of the heating element 48 are adjusted to obtain a resistance R adapted to an external current source supplying the heating element with the current I. Power (IR) is dissipated in the resistive heating element 48 by the Joule effect to heat the sensor 40. Accordingly, the heating element 48 loops around the sensor 40 at its periphery in a simple or snaking configuration as required for efficient distribution of the heat generated by the current I. Integrated micro-thermometer 49: Thin film portion 49 constitute a micro- thermometer integrated to the thin film C0 2 sensor 40. As illustrated in Figure 4, the thin film portion 49 defines an elongate wire 59 having a temperature- dependent resistance to enable measurement of the temperature of the sensor 40. Ionic conductor 44: A thin film 44 of NASICON (Na 3 Zr 2 Si 2 P0 12 ) is deposited on the thin film reference electrode 46 and to a portion of the surface of the thin film of dielectric material 42 between the electrodes 46 and 47 to constitute a thin film ionic conductor 44. The film 44 may be of a homogeneous composition, or may include a plurality of layers which through their composition or structure serve to act as passivation, stabilisation and/or diffusion-barrier films. Reactive CO., sensitive layer 45: The last thin film 45, deposited on the electrode 47, the surface portion of the thin film of dielectric material 42 between the metallic electrode 47 and the thin film ionic conductor 44, is made of sodium carbonate alone or stabilized by barium carbonate to form a reactive thin film C0 2 sensitive layer. Mechanism of detection of sensor 40: The mechanism of detection used by the sensor 40 is exactly the same as described in relation to the sensor 10 and, therefore, will not be further described in the present description. Fabrication of sensor 40: The different steps conducted during fabrication ooff tthhee thin film C0 2 sensor 40 are explained hereinafter. The surface 62 of the substrate 41 is first chemically etched to clean this surface 62 prior to thin film deposition. The substrate 41 must be properly cleaned to avoid organic and inorganic contamination (to levels of p.p.m.) of the thin layer 42 to be deposited. Such contamination may lead to reliability problems such as slow degradation of the sensor 40. The thin film of dielectric material 42 is then deposited on the substrate 41 either by RF reactive magnetron sputtering, or plasma enhanced or normal chemical vapour deposition. When the substrate is made of silicon, thermal oxidation of the silicon may be used to produce the insulating thin film 42 directly on the surface of the substrate 41. Platinum or another noble metal is deposited onto the thin film of dielectric material 42 to form the thin film reference electrode 46, second thin film metallic electrode 47, heating element 48 and micro-thermometer 49. It can be deposited by thin film techniques including reactive evaporation in an ultrahigh vacuum chamber, RF reactive magnetron sputtering, electroplating, electroless plating, screen printing, or plasma enhanced or normal chemical vapour deposition. Use of proximity masking can be used during deposition to produce from the single thin film 43 the reference electrode 46, second metallic electrode 47, heating element 48 and micro-thermometer 49. Alternatively, laser ablation can be used to cut in the thin film 43 the two electrodes 46 and 47, the heating element 48 and the micro-thermometer 49. Ionic conductor material is sputtered onto the thin film reference electrode 46 and a portion of the surface of the thin film of dielectric material 42 between the two electrodes 46 and 47 to form the thin film ionic conductor 44. The method for depositing this thin film 44 is the same as for thin film 15 of sensor 10 (Figures 1 and 2) . Sodium carbonate alone or stabilized by barium carbonate is then deposited on the second thin film metallic electrode 47, the thin film ionic conductor 44 and the surface of the thin film 42 between the ionic conductor 44 and the electrode 47 to form a reactive thin film C0 2 sensitive layer. RF magnetron sputtering can be used to deposit thin film 45. Other methods such as sol-gel coating, reactive evaporation in an ultra high vacuum chamber, plasma enhanced or normal chemical vapour deposition, etc. can also be contemplated. To enable wiring of the sensor 40 using conventional bonding techniques, no material is deposited on surface portion 50 of the thin film reference electrode 46 whereby an electric wire 51 can be connected thereto. Also, no material is deposited on surface portion 52 of the second thin film metallic electrode 47 whereby an electric wire 53 (Figure 4) can be connected thereto. As no material is deposited on the thin film heating element 48 and micro- thermometer 49, electric wires 57 and 58 can be connected to the ends 55 and 56 of the heating element 48, respectively, and electric wires 60 and 61 can be connected to the respective ends of the integrated micro-thermometer 49. Proximity masking can be used during deposition of each thin film 42-45 to produce the structure shown in Figures 3 and 4. Laser ablation of the thin films 42-45 may also be used to give to the C0 2 sensor 40 the structure of Figures 3 and 4. Finally, the substrate 41 is cut by means of laser scribing, cleaving or diamond saw techniques, or a combination of these techniques, and the thin film C0 2 sensor 10 is then packaged using conventional hybrid packaging techniques. The thin film C0 2 sensor 10,40 according to the invention is capable of continuously detecting and monitoring low concentrations of C0 2 with a sensitivity of 100 p.p.m. or less. It uses a solid electrolyte and may be fabricated by means of thin film techniques. The sensor of the invention has numerous applications in the field of air quality control, biomedical monitoring and industrial processes and control. Several thin film C0 2 sensors 10,40 can be mounted in cascade to increase the amplitude level of the detected electromotive force and thereby improve the C0 2 detection sensitivity. Figure 5 illustrates two thin film sensors 40 (Figures 3 and 4) mounted in cascade. In this particular case, the thin film reference electrode of the first sensor and the second thin film metallic electrode of the second sensor form a common electrode 63. The peripheral thin film heating element 64 then surrounds the two sensors 40 mounted in cascade. Also, the thin film C0 2 sensor 10, 40 can be provided with an outer C0 2 permeable membrane. When such a sensor 10 is immersed in blood, the membrane enables passage of C0 2 through it to thereby allow the sensor 10 to sense the C0 2 concentration in this blood. An electronic system (not shown) dedicated to the conversion of the electromotive force (EMF) between the two metallic electrodes to chemical concentration of C0 2 , as well as to periodic self- verification of the performance of the sensor 10, regeneration of the ionic conductor thin film 15, thermostating the temperature of operation, auto- recalibration etc. can be integrated to the substrate 11 along with the sensor 10,40. A smart electrochemical gas sensor is then obtained. Last of all, it should be pointed out that the chemistry of the thin films 15,44 and 16,45 of the sensor 10,40 in accordance with the present invention can be modified to enable detection of the concentration of gases other than C0 2 Although the present invention has been described hereinabove by way of preferred embodiments thereof, these embodiments can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention.

Claims: WHAT IS CLAIMED IS: 1. A monolithic gas sensor comprising: first and second electrodes each made of a film of electrically conductive material; a reactive layer made of a film of material sensitive to the gas to be detected; and an ionic conductor being under the form of thin film to efficiently conduct ions therein; wherein said first and second electrodes, said reactive layer, and said thin film ionic conductor are deposited on each other to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between said first and second electrodes. 2. A monolithic gas sensor as recited in claim 1, further comprising an integrated heating element made of a film of electrically conductive material, said heating element being supplied with electric current to heat said sensor. 3. A monolithic gas sensor as recited in claim 2, wherein said heating element is a thin film heating element. 4. A monolithic gas sensor as recited in claim 2, further comprising an integrated thermometer formed of a film of electrically conductive material having a temperature-dependent resistance which can be converted to temperature. 5. A monolithic gas sensor as recited in claim 2, further comprising an integrated thermometer formed of a thin film of electrically conductive material having a temperature-dependent resistance which can be converted to temperature. 6. A monolithic gas sensor as recited in claim 1, comprising: a substrate; a thin film heating element made of electrically conductive material deposited on said substrate; a thin film of dielectric material deposited on said thin film heating element; a reference electrode consisting of a thin film of electrically conductive material deposited on said thin film of dielectric material, said reference electrode constituting said first electrode; said thin film ionic conductor deposited on said thin film reference electrode; said reactive gas sensitive layer consisting of a thin film of reactive gas sensitive material deposited on said thin film ionic conductor; and said second electrode formed of a thin film of electrically conductive material deposited on said thin film reactive gas sensitive electrode. 7. A monolithic gas sensor as recited in claim 6, further comprising another thin film of dielectric material deposited on said substrate, the thin film heating element being deposited on said other thin film of dielectric material. 8. A monolithic gas sensor as recited in claim 6, further comprising a thin film micro- thermometer consisting of a portion of the thin film of electrically conductive material constituting the reference electrode, said thin film portion forming the micro-thermometer having a temperature-dependent resistance than can be converted to temperature. 9. A monolithic gas sensor as recited in claim 1, comprising: a substrate; a thin film of dielectric material deposited on said substrate; a thin film of electrically conductive material deposited on said thin film of dielectric material and divided into three separate thin film portions respectively forming (a) said first electrode constituting a thin film reference electrode, (b) said second electrically conductive electrode, and (c) a thin film heating element; said thin film ionic conductor deposited onto said thin film reference electrode; and said reactive gas sensitive layer deposited in thin film on said second electrically conductive electrode and on said thin film ionic conductor. 10. A monolithic gas sensor as recited in claim 9, wherein said thin film of electrically conductive material comprises a fourth portion constituting a thin film micro-thermometer having a temperature-dependent resistance which can be converted to temperature. 11. A monolithic gas sensor as recited in claim 1, wherein said ionic conductor comprises NASICON. 12. A monolithic gas sensor as recited in claim 1, wherein said gas to be detected is C0 2 , said ionic conductor comprises NASICON of formula Na 3 Zr 2 Si 2 P0 12 , and said reactive gas sensitive layer comprises Na 2 C0 3 . 13. A monolithic gas sensor as recited in claim 1, comprising a plurality of said electrolytic cell mounted in cascade in order to improve gas detection sensitivity of said monolithic gas sensor. 14. A method of fabricating a monolithic gas sensor comprising the step of depositing onto a substrate (a) first and second thin film electrodes made of electrically conductive material, (b) a reactive thin film layer made of material sensitive to the gas to be detected, and (c) an ionic conductor under the form of thin film to efficiently conduct ions therein, wherein said depositing step comprises stacking said first and second thin film electrodes, said reactive thin film layer, and said thin film ionic conductor to form an electrolytic cell in which a chemical reaction involving as reagent the gas to be detected produces an electromotive force between said first and second thin film electrodes. 15. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising the step of depositing on said substrate a thin film heating element made of electrically conductive material, and the step of depositing on said substrate a micro-thermometer made of a thin film of electrically conductive material having a temperature- dependent resistance which can be converted to temperature. 16. A method of fabricating a monolithic gas sensor as recited in claim 14, wherein said depositing step comprises patterning at least one of said first and second thin film electrodes, said reactive thin film layer, and said thin film ionic conductor though proximity masking. 17. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising the step of patterning the stacked first and second thin film electrodes, reactive thin film layer, and thin film ionic conductor through laser ablation. 18. A method of fabricating a monolithic gas sensor as recited in claim 15, wherein said depositing step comprises patterning, by means of proximity masking, at least one of said first and second thin film electrodes, said reactive thin film layer, said thin film ionic conductor, said thin film heating element, and said thin film micro-thermometer. 19. A method of fabricating a monolithic gas sensor as recited in claim 15, further comprising the step of patterning, by means of laser ablation, the stacked first and second thin film electrodes, reactive thin film layer, and thin film ionic conductor, and said thin film heating element and thin film micro-thermometer. 20. A method of fabricating a monolithic gas sensor as recited in claim 14, further comprising a step of rapid thermal annealing of said thin film ionic conductor to favor crystalline microstructure formation and thereby improve ionic conductivity of the thin film ionic conductor.

Inventors: CURRIE JOHN F, LECOURS ANDRE, CURRIE, JOHN, F., LECOURS, ANDRE
Applicants: CAPTEURS CAPCO R & D INC, LES CAPTEURS CAPCO R. & D. INC.
Application year: 1994
Main classification: G01N27
IPC codes: G01N27